How to Calculate Net Carbs
Alas, there's no offical formula for calculating net carbs, and net carbs on packaged foods can be deceiving since some sugar alcohols are truly zero net carb, while others are not. But the general consensus among ketonians is the following: to calculate the net carbs of your food, you subtract the total amount of its fiber and sugar alcohols (such as xylitol and erythritol, which your body doesn't absorb) from the total carbs.
Here's the basic formula: Net carbohydrates = total carbohydrates - fiber - sugar alcohols (if applicable).
But as we mention above, sugar alcohols can confuse things if you don't know which ones count as carbs and which don't. But we're here to clear things up! Below are two lists of sugar alcohols. The sugars under the "Sugar Alcohols with Zero Carb Count" don't need to be factored into your net carb formula. The sugar alcohols under the "Sugar Alcohols with Some Carb Count" do need to be factored in, but only at 50% of their total carbs. That means that if a cookie with 10 total carbs contains 2 grams of fiber and 4 grams of sorbitol, the formula would be 10 grams total carbs - 4 grams fiber - 2 grams sorbitol = 4 grams net carbs.
Sugar Alcohols with Zero Carb Count
Sugar Alcohols with Some Carb Count
See! It's not so hard to figure out!
Here's an example using a medium avocado (which doesn't contain sugar alcohols): If the avocado contains 17.1 grrams of total carbs and 13.5 grams of fiber, you subtract the fiber (13.5 grams) from the total carbs (17.1 grams), which leaves you with 3.6 grams of net carbs. Talk about reason to enjoy guacamole!